The present invention relates to an optical switch which is disposed in an optical waveguide and, more particularly, to both a branch type optical switch which has a high coupling efficiency at a branching point of the optical waveguide and an excellent extinction ratio and an optical switch which uses the branch type optical switch so that it has an excellent extinction ratio and a small element length suitable for integration.
One of the branch type optical switches according to the prior art is disclosed, for example, by A. Himeno et al. in National Conference Record, 1985, Semiconductor Devices and Materials, The Institute of Electronics and Communication Engineers of Japan, S7-3, pp. 1-345 to 1-346. This optical switch has no region for changing the refractive index but makes a multi-mode optical waveguide coupling by increasing the width of the optical waveguide at each branching point of the waveguide so as to reduce the coupling loss at the branching point. However, no consideration is given to the coupling to a single-mode optical waveguide with a low loss.
Because of the lack of the refractive index changing portion in the optical waveguide, moreover, much crosstalk seems to occur in the waveguide light.
Moreover, the above-specified prior art gives no consideration to enhancement of the coupling efficiency of two single-mode optical waveguides for coupling one of input optical waveguides and an output optical waveguide but rather uses the multi-mode optical waveguide. This raises another problem of a low efficiency in the coupling with the subsequent single-mode optical waveguide such as a single-mode optical fiber.
The description thus far made is directed to an optical switch of the type having a branching point. An optical switch having two intersecting optical waveguides will be described in the following. The optical switch having this structure is detailed in not only (i) European Patent Laid-Open No. 0147195 but also: (ii) K. Ishida et al., Appl. Phys. Lett., vol. 50, No. 3, January 1987, pp. 141 to 142; (iii) H. Inoue et al., National Conference Record, 1985, Semiconductor Devices and Materials, The Institute of Electronics and Communication Engineers of Japan, S7-4, pp. 1-347; and (iv) K. Ishida et al., Autumn Meeting Record, 1984, The Institute of Applied Physics of Japan, 1p-L-16, pp. 147. The U.S. patent application Ser. No. 684,784 corresponding to the above-specified European Patent Laid-Open No. 0147195 is now pending. All the optical switches disclosed in those references are of the total reflection type, in which the refractive index of a total reflection portion is changed by changing the carrier concentration. Despite this fact, however, the intersection angle has a relatively small value such as 5 or 15 degrees in the Embodiments of the European Patent Laid-Open No. 0147195.
In the optical switch having two intersecting optical waveguides, an intersection angle near 90 degrees may be more desirable for reducing the size. This is desired especially in case a multiplicity of optical switches are arranged in a matrix form.
The corresponding matrix type optical switch is disclosed, for example, by M. Kondo et al., National Convention Record, 1985, The Institute of Electronics and Communication Engineers of Japan, 954, pp. 4 to 78. This optical switch does not make use of optical reflection but has its optical output periodically varied with an increase in the applied voltage so that it is unsuitable for integration because of much variability in the outputs of the individual switch elements on a substrate.